1. Field of the Invention
This invention relates generally to the inclusion of subsea manifolds while laying underwater pipelines when the inclusion of the manifolds at the conventional work area is made impractical due to space or operational considerations.
2. Description of the Related Art
Subsea manifolds are generally rectangular structures with a representative size for discussion being 12 feet wide by 10 feet tall by 25 feet long. In the long direction and approximately 3 feet above the bottom of the skid a main pipeline will pass through the skid. The main pipeline is not usually at the bottom of the skid because the main skid structural beams are at the bottom and any valves in the pipeline would have downward protrusions.
Subsea manifolds are typically for the purpose of making connections between short run and long run pipelines on the sea floor. The arrangement of pipelines on the ocean floor looks much like the arrangement of roads on a map. Individual subsea oil or gas completion pipelines (called flowlines) must tie into larger and longer pipelines to ultimately get the produced fluids to the shore. Although most subsea manifolds will be of the pipeline connection type, others include pig launchers, control skids, and pumping skids.
Subsea pipelines have most often been laid on the ocean floor by connection of welding on the deck of a barge, lowering off the back of the barge down a stinger, and then laying on the ocean floor. The curve down off the barge and then reverse curve onto the ocean floor gives a xe2x80x9cSxe2x80x9d bend in the pipeline during the lay process. The stinger allows the pipeline and its associated weight to be curved down toward the ocean floor without kinking or damaging the pipeline.
The stinger will involve several rollers to form a bending radius down to the angle necessary to go to the ocean floor. They are typically of a xe2x80x9cVxe2x80x9d shape, to allow them to be workable with a variety of pipeline sizes. It is inherent that it would be difficult for a skid package to go over the rollers in a xe2x80x9cSxe2x80x9d lay stinger. In addition to interferences with the rollers, there will be interferences with side structures in the pipe lay operations, and the long rigid section implied by the length of the manifold would substantially complicate the associated bending stresses while bending the pipe around the stinger.
In deeper water, it is advantageous to connect the pipeline vertically and lower it directly into the water, with a single bend at the ocean floor. This gives the shape of a xe2x80x9cJxe2x80x9d for a system known as J-Laying pipe. The xe2x80x9cJxe2x80x9d will not normally be vertical at the top, but rather must have an angle with a horizontal component to be able to pull horizontal tension on the pipeline as it is being laid onto the ocean floor. If it does not have horizontal tension during laying;, it will buckle as it is laid on the ocean floor and be damaged beyond use.
J-Lay towers characteristically have a relatively small hole through the main work area which is designed to engage and support the weight of the pipeline being run. Additionally they will have a stinger below the working area which is similar to the one on the xe2x80x9cSxe2x80x9d lay barge, except instead of bending in one direction, it will have full circle bending capability. When the J-Lay tower is properly aligned with the pipeline being laid below it, it will be lined up with the pipeline and not require the handling of a bending moment when installing a subsea manifold. However, J-Lay towers tend to be more restrictive in size openings to pass manifold skids than the xe2x80x9cSxe2x80x9d lay barges are.
The present invention is directed to overcoming, or at least reducing the effects of one or more of the problems set forth above.
The object of this invention is to provide a method whereby subsea manifolds can be installed on a section of a subsea pipeline after the subsea pipeline has passed the restrictive areas characteristic to the laying of the pipe.
A second object of this invention is to provide means for attaching subsea manifolds to pipelines after the portion of the pipeline to which the pipeline will be attached is already below sea level.
A third object of this invention is to provide means for preventing the pipeline connection point for attaching subsea manifolds from flooding before the manifold is attached.
Another object of this invention is to provide means to move sections of a pipeline including valves and tees past rollers on stingers without damaging the rollers.
Another object of this invention is to provide means to allow the removal of a subsea manifold from a subsea pipeline for replacement or repairs after it is in service.
Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon referring to the drawings which follow.